A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand,
Johannesburg, in fulfilment of the requirements for the degree of Master in Science in
Engineering,
Johannesburg September 1998 / Combustion modelling of utility furnace chambers provides a cost efficient means to
extrapolate the combustion behaviour of pulverised fuel (pf) as determined from drop
tube furnace (DTF) experiments to full scale plant by making use of computational fluid
dynamics (CFD). The combustion model will be used to assimilate essential
information for the evaluation and prediction of the effect of
• changing coal feedstocks
• proposed operational changes
• boiler modifications.
TRI comrnlssloned a DTF in 1989 which has to date been primarily used for the
comparative characterisation of coals in terms of combustion behaviour. An analysis of
the DTF results allows the determination of certain combustion parameters used to
define a mathematical model describing the rate at which the combustion reaction
takes place. This model has been incorporated into a reactor model which can
simulate the processes occurring in the furnace region of a boiler, thereby allowing the
extrapolation of the DTF determined combustion assessment to the full scale. This
provides information about combustion conditions in the boiler which in turn are used
in the evaluation of the furnace performance.
Extensive furnace testwork of one of Eskom's wall fired plant (Hendrina Unit 9) during
1996, intended to validate the model for the ar plications outlined above, included the
measurement {If :
• gas temperatures
• O2, C02, CO, NOx and S02 concentrations
• residence time distributions
• combustible matter in combustion residues extracted from the furnace
• furnace heat fluxes.
The coal used during the tests was sampled and subjected to a series of chemical and
other lab-scale analyses to determine the following:
• physical properties
• composition
• devolatilisation properties
" combustion properties
The same furnace was modelled using the University of Stuttgart's AIOLOS combustion
code, the results of Which are compared with the measured data.
A DTF derived combustion assessment of a coal sampled from the same site but from
a different part of the beneficiation plant, which was found to burn differently, was
subsequently used in a further simulation to assess the sensitivity of the model to char
combustion rate data. The results of these predictions are compared to the predictions
of the validation simulation.
It was found that the model produces results that compare well with the measured
data. Furthermore. the model was found to be sufficiently sensitive to reactivity
parameters of the coal. The model has thereby demonstrated that it can be used in the
envisaged application of extrapolating DTF reactivity assessments to full scale plant. In
using the model, it has become apparent that the evaluations of furnace modifications
and assessments of boiler operation lie well within the capabilities of the model. / MT2017
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/22714 |
| Date | January 1998 |
| Creators | Eichhorn, Niels Wilhelm |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (1 v. (various pagings)), application/pdf |
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